Quake clusters could be linked

A spate of deadly earthquakes in the last fortnight is challenging the belief it is merely a coincidence they occurred in quick succession, despite being far apart on the globe.

One quake caused the tsunami which killed more than 140 people in Samoa.

Just hours later, but thousands of kilometres away, another quake devastated the Indonesian city of Padang, killing thousands.

Geoscience Australia now says the cluster of quakes in the Pacific and thousands of kilometres away near Indonesia could be related.

That would mean the weak points in the Earth's crust are more susceptible than previously thought.

Dr Gary Gibson has been studying earthquakes since the 1970s and says he cannot remember having noticed so many over a couple of weeks.

"I can't recall in 35 years having had so many earthquakes that have attracted attention in such a short period," he said.

The occurrence of so many earthquakes so far apart geographically but so in such a short time has also raised eyebrows at the commonwealth agency Geoscience Australia.

"It is pretty unusual to get this many large earthquakes at such different locations," senior seismologist Dr Phil Cummins said.

The sudden movement caused by an earthquake is known to trigger other quakes nearby, but that effect is usually limited to a small area.

Dr Gibson says the quakes around West Sumatra, Samoa, Vanuatu and the Philippines are too far away to be linked in some way.

"Although the plates are fairly rigid, they are not that rigid and the affect of stress only goes out, as far as I can see, to hundreds of kilometres," he said.

Quake clusters

A cluster of big quakes would usually be dismissed as a coincidence if they are not close together. But the events of the last couple of weeks are not so easy to dismiss.

And it is more than just coincidence or conjecture that is challenging the understanding of how far afield earth tremors can have an impact.

Just last month, the journal Nature published findings linking the earthquake which caused the devastating Indian Ocean tsunami in 2004 with the San Andreas fault in California.

US seismologists found evidence parts of the San Andreas fault had been weakened by that quake and the ones that followed in the Sumatran region.

Dr Cummins says the recent quakes near Indonesia and in the Pacific could be linked in the same way.

"Most seismologists would never completely discount the possibility that they could be related but in the past, we thought it was very unlikely," he said.

"I think even now it would be virtually impossible to prove any connection between these earthquakes, simply because we don't have the data on fault strength that we need to do that.

That traditional wisdom that such far apart events cannot be linked and are more likely a coincidence is being challenged by the events of the last couple of weeks.

"I would have to say that many seismologists are rethinking this and considering more seriously the possibility that they could be linked," Dr Cummins said.

It is a revelation in a science which seems a little shaky at times. Dr Cummins says there is still some guess work involved and he cannot rule out that it could still be a coincidence.

"It is just incredible. Without the detailed data that we need to study things like fault properties, which we may never have, I don't think we will ever be able to decide the question either way. We will never know," he said.